Selective amino acid substitution reduces cytotoxicity of the antimicrobial peptide mastoparan

• Amino acid substitutions enhance MP antimicrobial potency and membrane selectivity.
• [I5, R8] MP adopts an amphipathic α-helix structure indicated by molecular modeling.
• [I5, R8] MP affects permeability and membrane potential of bacteria.
• High-resolution SEM-FEG images showed bacterial and fungal membrane damage.
• [I5, R8] MP caused rapid complete killing of E. coli within 15 min.

The emergence of antibiotic-resistant clinical isolates and the decreased rate of development of new antibiotics are a constant threat to human health. In this context, the therapeutic value of mastoparan (MP), a toxin from wasp venom, has been extensively studied. However, since MP shows significant cytotoxic activities, further optimization is needed. Here we evaluated the antimicrobial and cytolytic activities of an MP analog created by Ala-substitution in positions 5 and 8, named [I5, R8] mastoparan ([I5, R8] MP). We found that [I5, R8] MP displayed a broad-spectrum antimicrobial activity against bacteria and fungi (MIC in the range 3–25 μM), without being hemolytic or cytotoxic toward HEK-293 cells. In addition, [I5, R8] MP-amide was highly potent (MIC = 3 μM) against antibiotic-resistant bacteria. The interaction with microbial membranes was investigated revealing that [I5, R8] MP is able to form an active amphipathic α-helix conformation and to disturb membranes causing lysis and cell death. Based on our findings, we hypothesize that [I5, R8] MP follows a mechanism of action similar to that proposed for MP, where the pore-forming activity leads to cell death. Our results indicate that hydrophobic moment modified by amino acid substitution may enhance MP selectivity.

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